1. Field of the Invention
The present invention relates to an amorphous lithium nickelate positive active material containing cobalt as a lithium nickelate positive electrode and a producing method thereof.
2. Description of the Related Art
In recent years, with development of a portable electronic appliance, a high performance battery has been demanded. A lithium battery with a negative electrode of carbon and a positive electrode of lithium cobaltate which is composite oxide in layered structure has been put into practice as a non-aqueous battery having a high operating voltage and high energy density. However, LiCoO.sub.2 is expensive and poor in resources. For this reason, lithium containing manganese composite oxide or lithium nickelate has been proposed as an alternative material. However, the battery, which uses the lithium containing manganese composite oxide, has disadvantages of a low theoretical capacity density and great reduction in capacity with an advance of a charge/discharge cycle.
On the other hand, LiNiO.sub.2 (lithium containing oxide) is a layered compound having the same crystalline structure as that of LiCoO.sub.2 which has been put into practice and contains lithium inserted between the layers of the edge share of NiO.sub.6 octahedra. It can be generally fabricated in such a manner that a nickel source such as Ni(NO.sub.3).sub.2, Ni(OH).sub.2, NiCO.sub.3 or NiO, and NiOOH, etc. and a lithium source such as LiOH, LiNO.sub.3 or Li.sub.2 CO.sub.3 and Li.sub.2 O.sub.2, etc. are mixed and the mixture thus formed is heat treated in an atmosphere of about 600.degree. C. to 900.degree. C.
As reported in "Solid State Ionics", 44, 87, 1990; "Chem. Express. 7, 689, 1992; or "Dai 33 Kai Denchi Tooronkai Kooen Youshi Shuu", its structure is similar to the structure of rock salt so that nickel is replaced by lithium ions to provide a disordered structure, thus reducing the capacity. In order to obviate such difficulty, an attempt to use nickel oxyhydroxide as the nickel material has been made. For example, Unexamined Japanese Patent Publication (kokai) No. Sho. 63-19760 proposes to use nickel oxyhydroxide containing Co of 20 to 75% as an active material for a lithium battery.
Further, in order to improve the discharging characteristic, Unexamined Japanese Patent Publication (kokai) No. Hei. 6-31045 proposes to mix hydroxide containing or oxide containing trivalent nickel ions with lithium salt and heat-treat the mixture thus formed. According to this proposal, hypochlorite solution, chlorine-containing solution or bromine containing solution is reacted with bivalent nickel hydroxide dispersed in sodium hydroxide (NaOH) solution to fabricate nickel oxyhydroxide. The hydroxide or oxide containing nickel oxyhydroxide is mixed with lithium nitrate. The mixture is pressurized, molded or dried and heated in air at 600.degree. C. to 800.degree. C. The resultant mold is crushed and molded again, and heat-sintered in air at 700.degree. C. to 900.degree. C. to create lithium nickelate.
However, the above process has the disadvantage that it was difficult to create pure lithium oxide nickel; the voltage of the charging/discharging characteristic varies at plural stages, e.g. four stages; and the high rate discharge performance is reduced.
On the other hand, the synthesis process of lithium nickelate not by a chemical method but an electro-chemical method is disclosed in Soviet Electrochem., 6, 1268, 1970, GS News 37, 84 (1978) and GS News 45, 23 (1986). But these references only describe the electrode behavior regarding an alkaline battery.
An application of such lithium nickelate fabricated electrochemically to the lithium battery is disclosed in Unexamined Japanese Patent Publication (kokai) No. Sho. 63-19761. This reference proposes to use nickel hydroxide charged in a lithium hydroxide solution as an active material. The process of its fabrication is likely to become complicated so that the stabilized active material cannot be obtained as the case may be.
Unexamined Japanese Patent Publication (kokai) No. Sho. 63-19760 reports that trivalent or lower oxidizing state of nickel can be discharged in an non-aqueous solution, e.g. nickel oxyhydroxide (trivalent oxidizing state of nickel) containing cobalt with lithium ions inserted is discharged at 2.7 to 2.4 V.
Solid State Ionics 44, 87 (1990) reports that although LiNiO.sub.2 can be discharged to become Li.sub.2 NiO.sub.2, its discharging potential varies at several stages so that LiNiO.sub.2 in a hexagonal structure becomes Li.sub.2 NiO.sub.2 in a rhombic structure. However, this reference does not clarify whether or not the positive electrode active material has high capacity.
Generally, the capacity of the conventional nickel oxide lithium is at most about 200 mAH/g. In addition, the theoretical capacity density is considered 275 mAh/g based on one electron change reaction of Equation (1) so that implementation of its higher capacity has been demanded. ##STR1##
As described above, generally, lithium nickelate can be synthesized by mixing nickel compound with lithium compound and baking the mixture in an oxidizing atmosphere of 600.degree. C. to 900.degree. C. The irregular arrangement of the crystalline structure occurring in such a high temperature atmosphere is a very serious problem.
Nickel oxide has serious disadvantages that its charging/discharging voltage characteristic varies at plural stages, e.g. four stages, and high rate discharge performance is lowered. For this reason, it cannot be an alternative of lithium cobaltate having the same layered structure. When viewed from the standpoint of electrode, this is probably because lithium nickelate is difficult to diffuse lithium ions attendant on charging/discharging reaction and the diffusion does not occur uniformly.
In order to obtain lithium nickelate having a uniform structure, attempts of synthesizing at a lower temperature have been made. However, its solid-state baking, whose reactivity becomes low, is difficult to provide a uniform sample. Therefore, the low temperature synthesizing which is different from the solid-state baking and is based on a simplified process can be demanded.
From the standpoint of cost, although the low temperature synthesizing is preferred, a useful synthesizing has not been established at present.
Further, lithium nickelate when synthesized by reacting lithium nitrate with nickel oxyhydroxide containing cobalt and when providing a uniform charging/discharging reaction can only give a dissatisfactory discharging capacity of at most about 200 mAh/g.